1. Field of the Invention
The present invention relates to atomic clocks and, particularly, to trapped mercury ion atomic clocks having a co-magnetometer.
2. Description of the Background
Atomic clocks are used in a variety of applications that require superior stability and reliability with continuous operations, including communications (frequency stabilization), navigation (GPS), astronomical observation (radio astronomy arrays), national timekeeping, and space navigation. The quality of a clock depends on its stability and accuracy. A stable clock is based on a constant, unchanging output frequency whose fluctuations are small, but whose absolute value may not be well known. An accurate clock is based on a frequency whose absolute value is well known.
In 1991, NASA demonstrated a new trapped mercury ion atomic clock that relies on an energy transition of trapped mercury ions (a mercury atom that is missing one electron). See J. D. Prestage, G. J. Dick, and L. Maleki, “Linear Ion Trap Based Atomic Frequency Standard,” IEEE Trans. Instrum. and Meas. 40, pp. 132-136 (1991). Subsequently in 2002, NASA demonstrated an enhanced version of this clock that used two separated traps and shuttled ions between them. This trapped ion atomic clock is detailed in U.S. Pat. No. 5,420,549 to Prestage, which is incorporated by reference in its entirety. The '549 patent describes linear ion traps having either a single trap region or two trap regions. Generally, these linear ion traps are rf Paul traps that comprise multiple molybdenum rods to which an rf voltage is applied between rod pairs to create a trap well. The linear ion traps are generally used with mercury ions such as 199Hg+.
FIG. 1 is a simplified energy level diagram for 199Hg+. Using the linear ion trap in an atomic frequency standard, the 199Hg+ ions are created from a neutral mercury vapor by pulsing an electron beam along the trap axis and then collisionally cooling the trapped 199Hg+ ions with a background buffer gas such as helium or neon. During state selection and detection, a 202Hg+ discharge lamp generates ultraviolet light at 194 nm to optically pump the trapped 199Hg+ ions into the S1/2, F=0, mF=0 ground state hyperfine level, by driving the S1/2, F=1 to P1/2, F=1 transition. The optically excited 199Hg+ ions eventually decay back into the desired ground state hyperfine level, producing fluorescence. The discharge lamp is then dimmed to avoid an AC Stark shift, and an interrogating microwave field at approximately 40.507 GHz, derived from a local oscillator, drives the S1/2, F=0, mF=0 to S1/2, F=1, mF=0 clock transition. After interrogation, the lamp is again brightened and fluorescence given off by ions excited by this light is used for state detection. On each clock cycle, a small number of ions are lost from the trap so just after state detection, the electron beam is briefly pulsed on to replace these lost ions. The lamp is left in its bright state during this reloading phase to prepare the existing and new ions as before by optically pumping them back to the S1/2, F=0, mF=0 ground state hyperfine level. An optical detector, such as a photomultiplier tube, measures the fluorescence during state detection. The detected fluorescence indicates the degree to which ions were excited during microwave interrogation and therefore the degree to which the microwave interrogation frequency was on resonance with the clock transition. A coil is used to generate the quantization axis magnetic field (C-field).
The frequency of the interrogating microwave field is modulated from the clock transition frequency, 40.507 GHz, with an alternating offset frequency, ±Δv, causing a corresponding modulation of fluorescence. The frequency of the oscillator is subsequently adjusted to null the difference in light fluorescence obtained at the ±Δv offsets. The condition of null fluorescence occurs when the multiplied output of the oscillator is centered on the atomic resonance, because frequency detuning to ±Δv will give equal fluorescence levels. The output signal of the oscillator provides a stable frequency reference to be used as the basis for an atomic clock.
The stability of the 199Hg+ trapped ion atomic clocks, however, can be degraded by environmental perturbations such as thermal changes, magnetic fields, radiation, and acceleration. Accordingly, 199Hg+ trapped ion atomic clocks are designed to be insensitive to these environmental perturbations, especially magnetic field variations. In terrestrial applications having minor variations in magnetic field, magnetic effects can be virtually eliminated in 199Hg+ trapped ion atomic clocks by surrounding the clock with passive magnetic shields fabricated from a highly permeable nickel-iron alloy. In space, however, atomic clocks can be subject to much larger fluctuations in local magnetic fields that have the potential to greatly influence atomic transition frequencies and thus degrade clock stability, and passive magnetic shielding alone may be inadequate to prevent significant frequency fluctuations in the clock, or may exceed mass and volume restrictions that are critical for space navigation.
Accurate real time measurement of magnetic fields near or inside the ion trap would permit compensation for internal variations of the clock transition that would allow use of conventional shields or even reduction or elimination of the shielding. Using electronic magnetometers, such as a fluxgate magnetometer, does not solve this problem, however, because they generate their own perturbing electromagnetic fields, which prevents locating the magnetometer close enough to the trapped ions to determine the magnetic field experienced by the ions. Another potential solution is using the trapped 199Hg+ ions themselves to measure the magnetic field environment by monitoring shifts on field sensitive, Zeeman transitions, but this method too degrades clock performance because such measurements must be made in place of some clock transition interrogations, thereby increasing the time required to average to a given stability level.
An atomic energy level is field sensitive (first order sensitivity) if the quantum number, mF, is not equal to zero, mF≠0. Conversely, an atomic energy level is field insensitive (second order sensitivity or less) if the magnetic quantum number is equal to zero. The change between magnetic quantum numbers, ΔmF, for a given transition determines the polarization required of the incident microwave field used to drive the transition. If ΔmF=0, for example, the 199Hg+ clock transition from S1/2, F=0, mF=0 to S1/2, F=1, mF=0, the microwave polarization must be parallel to the quantization axis defined by the C-field, which here is parallel to the long axis of the trap, to drive the transition. If ΔmF≠0, the polarization must be perpendicular to the quantization axis. Essentially due to the different characteristics of trapped ion motion parallel and perpendicular to the quantization axis, transitions that are excited by microwaves with polarization parallel to the trap axis (ΔmF=0) are Doppler free, while transitions excited by microwaves with polarization perpendicular to the trap axis (ΔmF≠0) are Doppler broadened. The frequency of Doppler broadened transitions is less certain and this frequency uncertainty is highly temperature dependent. There are no field sensitive (mF≠0), ΔmF=0 hyperfine transitions for 199Hg+ ions, and thus to probe a field sensitive transition without Doppler broadening, the ion trap and the microwave polarization must be rotated relative to the C-field. Additionally, interrogation of the field-sensitive transition must be sequential to the clock transition interrogation, which increases the clock cycle time and thus degrades clock performance.
Thus, there is a need for a method and apparatus for measuring the local magnetic field strength within an ion trap without degrading trapped ion atomic clock performance.